1. Field of the Invention
Cartridge-type heating elements.
2. Prior Art
The utilization of thermostatic elements to control heat is, of course, well known and widely practiced in virtually every area where temperature control is necessary or desirable. In most instances, the thermostatic control of the electrical circuit for the heating element is located at a point which is remote from the heating element itself and serves to interrupt the flow of electrical heating current to the heating element.
In the cartridge heater field where such heating elements are used in immersion applications or for the conduction heating of solids where either control or overheating protection of the system is required, various attempts have been made to provide for simple and inexpensive thermostatically controlled heating elements.
One example, which has been utilized in the past, is the thermal fuse type in which one branch of the circuit to the heating element includes a solder type of joint which has a predetermined melting point. If the temperature at the solder point exceeds the predetermined limit, the solder melts opening the circuit and thus permanently disabling the heater.
In another successful product, an adjustable thermostat has been positioned internally near the top of a cartridge-type heater. In practice, the thermostat is designed to have adjustable set points by means of a shaft extending out of the cartridge heater which, in turn, rotates to mechanically vary the position of bi-metallic contacts and thus alter the temperature at which the circuit to the heating element is interrupted. Such adjustable thermostatically controlled cartridge heaters are very expensive and difficult to manufacture. In order to achieve the necessary precision of control, extensive hand labor, involving many individual steps, are involved. Of necessity, the material costs are excessive for widespread applications. Thus, the adjustable thermostat itself, along with the hermetic seals and related structure permitting the thermostat to be adjusted externally are all expensive elements. In order to assemble the adjustable thermostatically controlled cartridge heater element, the adjustable thermostat and its control shaft along with the electrical leads must be first assembled and then inserted into the sheath in such a manner as to not disturb the previously aligned insulation discs and heater elements and leads which must be first positioned in the lower portion of the sheath.
A still further example of the prior art thermostatically controlled cartridge-type heaters utilized a snap disc thermostat placed in the bottom of a tube or sheath of a cartridge-type heater. Such thermostats are expensive and are of necessity relatively large in diameter. In addition, by nature, their operation depends on a large surface area of contact which means that one face of the thermostat must be in contact with the bottom end of the cartridge-type heater. Due to size limitations, such snap disc thermostat elements could only be positioned in heaters of the cartridge-type which are at least three-quarters of an inch in diameter or larger. The leads to the thermostat are attached to one of the heater coils fitted into a ceramic core element. Of necessity, the specific type of cartridge heater which could utilize a snap disc thermostat is of the low watt density type since the higher watt density types, which require packing the sheath with a compressed quantity of magnesium oxide, was not possible without interfering with the preset temperature at which the snap disc thermostat would open the circuit to the heating coils.